1. Field of the Invention
The present invention relates generally to a ceramic honeycomb structure, a process of producing the honeycomb structure, and a coating material used in the structure. The invention is particularly concerned with a technique for effectively reinforcing the ceramic honeycomb structure and facilitating production thereof, assuring easy and wide application of the honeycomb structure produced.
2. Discussion of the Prior Art
In recent years, it is under discussion to intensify automobile emission regulations to meet a growing demand for preventing air pollution. At present, a catalytic converter having a ceramic honeycomb structure as a catalyst substrate or support is utilized for purifying automobile exhaust gas or emissions. The ceramic honeycomb structure is formed by extrusion as an integral body with a multiplicity of through-holes or cells defined by honeycomb thin walls. As a measure to further improve the catalytic conversion or purification efficiency of the catalytic converter, it has been proposed to improve a so-called warm-up characteristic of the converter, namely, to enhance the catalytic activity from the beginning of operation of the converter, by reducing the heat capacity of the honeycomb structure and thereby shortening the time required for heating the structure to a sufficiently high operating temperature.
In order to reduce the heat capacity of the ceramic honeycomb structure, there is a need to reduce the weight or bulk or apparent density of the honeycomb structure without changing its geometric surface area. To meet this need, it is proposed to reduce the thickness of the walls or webs defining the cells or to increase the open porosity of the honeycomb structure. As the easiest way to enhance the catalytic conversion efficiency, it is also proposed to increase the area of the honeycomb structure which supports the catalyst, namely, the volume of the honeycomb structure. In the automobile application, however, it is difficult to change the area or space in an automobile for installation of the converter. If a plurality of honeycomb structures are connected in series to thereby increase the total volume of the honeycomb structures, the resistance to flow of exhaust gas from an automobile engine is undesirably increased with a result of a reduced engine power, for example. To improve the catalytic conversion efficiency, therefore, it is preferred to reduce the thickness of the walls partitioning the cells of the honeycomb structure and increase the open porosity of the structure, so as to increase the volume or catalyst-bearing area of the honeycomb catalyst support without increasing the resistance to the exhaust gas flow.
On the other hand, exhaust gases emitted by a diesel engine car are purified in terms of particulates emitted particularly from the diesel engine, as well as NOx, CO, and HC which are also emitted from an ordinary gasoline engine car. In purifying the exhaust gas from the diesel engine, therefore, a diesel particulate filter (DPF) is employed to remove the particulates while the honeycomb structure is employed to remove NOx and others by a three-way catalytic conversion. Since a relatively large amount and high concentration of exhaust gases are emitted by the type of vehicles, such as large-sized buses and trucks, in which diesel engines are installed, a sufficiently large-sized honeycomb structure having an outside diameter of as large as 300 mm is needed to purify the exhaust gases in the manner as described above.
All of the above-described measures to effectively control the exhaust emissions, such as reduced thickness of the honeycomb walls, and lowered bluk of the honeycomb structure due to increased open porosity thereof, result in reduction in the mechanical strength of the honeycomb structure, and thereby cause various problems to the structure. For example, it is extremely difficult to achieve sufficiently reduced thickness of the honeycomb walls from the standpoint of production engineering. Upon extrusion molding of the thin-walled honeycomb structure, the extruding rate or speed of a clay varies depending upon portions of an extrusion die from which the clay is extruded, and an outer peripheral portion of the honeycomb structure (green body) may suffer from distortion or deformation of the cells, or cracks in an outer wall of the structure. The thus extruded honeycomb body has a low mechanical strength and may therefore suffer from breakage or deformation of the cells due to its own weight, which results in lowered dimensional accuracy of the resulting honeycomb product. Since a portion of the honeycomb structure having such defective cells is likely to be broken at the early period of use of the structure, due to the lower mechanical strength thereof compared to the other portions, it is necessary to remove the defects in the cells to assure a sufficiently high strength of the thin-walled honeycomb structure as a whole. Even if the thin-walled honeycomb structure consists of normal cells which do not include distorted or deformed cells of low mechanical strength and has an integrally formed outer wall which is free from cracks, such a honeycomb structure is still unsatisfactory in its isostatic strength (i.e., strength to endure uniform gripping force exerted on the outer wall) when the structure is subjected to canning. This makes it necessary to provide a reinforcing member on the outer wall of the structure.
When the honeycomb structure is large-sized to achieve a diameter of about 300 mm, so as to provide a large-sized catalyst support or DPF, it is difficult to form by molding an outer wall having a uniform thickness as an integral part of the structure. In addition, the extruded green body of the honeycomb structure is poor in its ability to keep its shape due to its considerably low mechanical strength, and suffers from breakage or deformation due to its own weight, resulting in poor dimensional accuracy. In particular, an outer peripheral portion of the honeycomb structure has an extremely low mechanical strength.
In view of the above situations, it is proposed in JP-B2-51-44713 to cover the outer periphery of the honeycomb structure with a mixture of sodium silicate and zirconium silicate, in order to reinforce the structure. For the same purpose, a water-repellent reinforcing refractories may be provided on the outer circumferential surface of the honeycomb structure, as disclosed in Publication No. 50-48858 of unexamined Japanese Utility Model Application (JP-U-50-48858). It is also proposed to provide a glaze coating on the outer circumferential surface of the honeycomb structure, as disclosed in JP-U-53-133860. It is further proposed in JP-A-56-129042 assigned to the assignee of the present application to fill passages or through-holes in an outer peripheral portion of a honeycomb support with a suitable ceramic material, so as to strengthen the outer peripheral portion. In JP-U-63-144836 also assigned to the present assignee, it is proposed to provide a covering layer as reinforcing means on the outer wall of the honeycomb structure so as to compensate for a difference between the actual diameter and an intended diameter of the structure.
However, the known reinforcing means provided on the outer periphery of the honeycomb structure may be unsatisfactory in their reinforcing effects, or may have poor heat-resistance properties. The covering layer indicated just above tends to peel off or form cracks therein, for example. Thus, none of the known honeycomb structures is satisfactory in all terms of its mechanical strength, heat-resistance, thermal shock resistance and operating reliability, to the extent required for the structure to appropriately serve as a honeycomb catalyst support for purifying automobile exhaust gases.